UV map not applying to certain faces
That part of the UV map is not assigned to an image. If you just select the face, you'll notice that the image in the UV/image editor disappears.
Select the face and then select the image in the UV/image editor to "assign" it. To be sure of this not happening again, you should probably do this with the entire mesh selected (A).

Note that with proper materials/textures setup, this won't affect the render.



Also see How is this blendfile hiding textures?